Method for improving the air release of a lubricating oil

ABSTRACT

The invention provides a method for improving the air release of a lubricating oil as measured by ASTM D3427. The method comprises a step of mixing an additive into the lubricating oil.

FIELD OF THE INVENTION

The invention relates to a method for improving the air release of alubricating oil.

BACKGROUND OF THE INVENTION

Lubricating oils are used to protect and lubricate surfaces in contact,and may also be used to transmit power. It is known that air entrainmentcan reduce the efficacy of lubricating oils. Air entrainment is aphenomenon wherein air bubbles (typically having a diameter of less than1 mm) are dispersed throughout the lubricating oil. Entrained air can bedistinguished from free air (a pocket of air trapped in part of thesystem), from dissolved air (lubricating oils may contain between 6 and12 percent by volume of dissolved air) and from foam (air bubblestypically greater than 1 mm in diameter that congregate on the surfaceof the oil). Air entrainment can have a number of negative consequencesincluding loss of lubricity, possible oxidation of the lubricating oil,noisy operation, lower efficiency and higher oil temperatures.

The air entrainment properties of a lubricating oil are typicallymeasured using the ASTM D3427 air release test. This test measures thetime needed for air entrained in the oil to reduce in volume to 0.2%under the test conditions and at the specified temperature.

The present inventors have sought to improve the air entrainmentproperties of lubricating oils.

SUMMARY OF THE INVENTION

Accordingly, the invention provides a method for improving the airrelease of a lubricating oil as measured by ASTM D3427, said methodcomprising mixing an additive into the lubricating oil using a highshear mixer; wherein the additive is an alkyl acrylate polymer.

Such alkyl acrylate polymers have been previously used as anti-foamadditives. Air release times are typically considered to get worse withthe addition of additives, especially anti-foams such as silicone oils.This is explained by Duncanson in “Properties on Foam in LubricatingOils”, pages 9-13, May 2003 Lubrication Engineering, Journal of theSociety of Tribologists and Lubrication Engineers.

It is surprising that incorporating the alkyl acrylate polymer into thelubricating oil with a high shear mixer affects the air releaseproperties of the lubricating oil. With the present invention theskilled person can use the alkyl acrylate polymer to improve the airrelease of a lubricating oil.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides a method for improving the air release ofa lubricating oil as measured by ASTM D3427. In the test, compressed airis blown through the lubricating oil, which has been heated to atemperature of 50° C. After the air flow is stopped, the time requiredfor the air entrained in the oil to reduce in volume to 0.2% is recordedas the air release time. A desirable air release value is typically lessthan 3 minutes, preferably less than 60 seconds and most preferably lessthan 20 seconds.

The air release is improved as compared to air release achieved usingthe lubricating oil wherein the additive has not been mixed with thelubricating oil using a high shear mixer. The inventors have founduseful reductions in air release times when carrying out the method ofthe invention.

The lubricating oil comprises a base oil component, which may be a blendof several base oils. The base oils in the base oil component may be anyconventional base oils chosen from Groups I, II, III, IV or V of the API(American Petroleum Institute) base oil categories.

Preferably the base oil component comprises a GTL base oil, and morepreferably the base oil component comprises at least 75 wt % of a GTLbase oil based upon the weight of the base oil component. GTL base oilsare synthesised by the Fischer-Tropsch method of converting natural gasto liquid fuel. They have a very low sulphur content and aromaticcontent compared with mineral oil base oils refined from crude oil andhave a very high paraffin constituent ratio.

The lubricating oil suitably comprises additives. Preferably the amountof additives is less than 10 wt % based upon the weight of thelubricating oil, more preferably less than 5 wt %, most preferably lessthan 2.5 wt %. The additives may include antioxidants, antiwearadditives, demulsifiers, emulsifiers, rust and corrosion inhibitors, VIimprovers and/or friction modifiers.

Preferably, the lubricating oil has a kinematic viscosity in the rangeof from 5 to 220 cSt at 40° C., more preferably of from 10 to 200 cSt,most preferably of from 20 to 100 cSt. The kinematic viscosity issuitably measured using ASTM D445 (ASTM D7042).

In a preferred embodiment of the invention, the lubricating oil isformulated for use a hydraulic fluid. Hydraulic fluids are used in manydifferent types of hydraulic machinery and are used not only tolubricate the machinery but also to transmit pressure. Air entrainmentcan be a particular issue in hydraulic systems, causing spongy orerratic operation of the hydraulics.

In the method of the invention an alkyl acrylate polymer additive ismixed into the lubricating oil. Suitable alkyl acrylate polymers aredisclosed in, for example, U.S. Pat. No. 3,166,508. The molecular weightof the polymer is suitably less than 10,000, preferably less than 7,500and more preferably less than 5,000. In one embodiment the alkylacrylate polymer may be a homopolymer of an alkyl acrylate having atleast 3 but less than 7 carbon atoms in the alkyl radical. In anotherembodiment the alkyl acrylate polymer may be a copolymer of at least twodifferent alkyl acrylates in which the alkyl radical has from 1 to 18carbon atoms and in which the average number of carbon atoms in thealkyl radicals of the copolymer molecular is at least 3 but less than 7on a molar basis.

Suitably at least 0.0005 wt % of alkyl acrylate polymer, based upon theweight of the lubricating oil, is mixed into the lubricating oil,preferably at least 0.001 wt %. Suitably less than 0.1 wt % of alkylacrylate polymer, based upon the weight of the lubricating oil, is mixedinto the lubricating oil, preferably less than 0.01 wt %. The preferredamount of alkyl acrylate polymer will be a balance between using enoughto achieve the desired improvement in air release properties andavoiding unnecessary expense by using too much alkyl acrylate polymer.

The alkyl acrylate polymer additive is mixed into the lubricating oilusing a high shear mixer. The term “high shear mixer” is well-known topersons skilled in the art. A rotating impeller or high-speed rotorcreates flow and shear within the liquid that is mixed. In someembodiments the rotor will turn within a stationary component known as astator. Preferably the additive is mixed into the lubricating oil at arate of at least 400 rpm. The rate is preferably at least 600 rpm andmore preferably at least 800 rpm. Preferably the mixing is for at least10 minutes, more preferably at least 30 minutes, and most preferably atleast 60 minutes.

The invention is further explained in detail below by means of examples,but the invention is in no way limited by these examples.

EXAMPLES

Seven different base oil blends were prepared using combinations ofthree base oils:

-   -   1) Chevron 220 R (a Group II mineral oil available from Chevron)    -   2) XHVI 8 (a Fischer-Tropsch derived oil available from Shell)    -   3) Spectrasyn 8 (a poly-alpha olefin fluid available from        ExxonMobil)        Table 1 gives the amounts of each base oil (in weight % based        upon the total weight of the blend) present in each blend. The        blends in Table 1 are all ISO 46.

TABLE 1 Chevron 220 R XHVI 8 Spectrasyn 8 Blend 1 15 80 5 Blend 2 25 705 Blend 3 15 75 10 Blend 4 20.0 72.5 7.5 Blend 5 20 80 0 Blend 6 25 75 0Blend 7 0 100 0

An additive package containing extreme pressure/antiwear additive,friction modifier, ashless rust inhibitor, emulsifier and antioxidantwas added to the base oil blends to provide fully formulated lubricatingoils. The additive package was the same in each instance. Then 125 ppmof PC 3144 additive (available from Allnex), which is a 40% solution ofa modified acrylic polymer in a hydrocarbon solvent, was added to theformulations at 40° C. either at a stirring rate of 1200 rpm using anoverhead Caframo high shear mixer for an hour or at a stirring rate of300 rpm using a magnetic stirrer for an hour.

The air release time of the base oil blends, of the fully formulatedoils and of the fully formulated oils plus PC 3144 was tested using theASTM D3427 method. The results are given in Table 2:

TABLE 2 Air release time at 50° C. (seconds) Fully Fully formulated oilformulated oil Base Fully plus PC 3144 plus PC 3144 oil formulated withhigh with magnetic blend oil shear mixer stirrer Blend 1 5 16 5 Notmeasured Blend 2 31 31 5 Not measured Blend 3 5 5 5 Not measured Blend 420 26 5 Not measured Blend 5 5 31 16 10 Blend 6 15 10 5 19.8 Blend 7 5 55 10

The results show that air release of the base oil blends varies. Addingthe additive package can further increase the air release time. Addingthe PC 3144 additive using a high shear mixer can substantially reducethe air release time (unless the air release time is already very low aswas the case for Blend 3 and Blend 7). Adding the PC 3144 additive witha magnetic stirrer does not seem to have the same effect.

1. A method for improving the air release of a lubricating oil asmeasured by ASTM D3427, said method comprising mixing an additive intothe lubricating oil using a high shear mixer; wherein the additive is analkyl acrylate polymer.
 2. The method according to claim 1, wherein thelubricating oil comprises a base oil component, and the base oilcomponent comprises at least 75 wt % of a GTL base oil based upon theweight of the base oil component.
 3. A The method according to claim 1,wherein the lubricating oil has a kinematic viscosity in the range offrom 5 to 220 cSt at 40° C.
 4. A The method according to claim 1,wherein the lubricating oil is formulated for use a hydraulic fluid. 5.A The method according to claim 1, wherein the molecular weight of thealkyl acrylate polymer is less than 10,000.
 6. A The method according toclaim 1, wherein the alkyl acrylate polymer is a homopolymer of an alkylacrylate having at least 3 but less than 7 carbon atoms in the alkylradical, or the alkyl acrylate polymer is a copolymer of at least twodifferent alkyl acrylates in which the alkyl radical has from 1 to 18carbon atoms and in which the average number of carbon atoms in thealkyl radicals of the copolymer molecular is at least 3 but less than 7on a molar basis.
 7. A The method according to claim 1, wherein at least0.0005 wt % and less than 0.1 wt % of alkyl acrylate polymer, based uponthe weight of the lubricating oil, is mixed into the lubricating oil. 8.A The method according to claim 1, wherein the additive is mixed intothe lubricating oil at a rate of at least 400 rpm.
 9. A The methodaccording to claim 1, wherein the additive is mixed into the lubricatingoil at a rate of at least 800 rpm.